The invention relates to a diaphragm control circuit for a TTL (through-the-lens) automatic electronic flash, and more particularly, to a diaphragm control circuit for a TTL automatic electronic flash which may be used in a camera capable of photometry of reflection from a film surface and also having the capability to control the emission of light from the automatic electronic flash and in which a diaphragm control is enabled also during a photographing operation which takes place with the aid of the automatic electronic flash.
In a conventional combination of a single lens reflex camera and an automatic electronic flash, a value of diaphragm aperture specified by the electronic flash is automatically preset in the camera in response to a signal from the electronic flash such as a charging complete signal, and an exposure period is in fixed synchronized relationship with the operation of the electronic flash. Since the use of the electronic flash is required by a low level of illumination, which results in an insufficient amount of light incident to cause a dissatisfactory photometry, the diaphragm within a taking lens is fixed to an open value. However, when a picture is taken with the aid of the electronic flash with an open diaphragm, the depth of field will be greatly reduced to cause an "out-of-focus" picture even though the amount of light passing "through-the-lens" is maximized to increase the dynamic range of emission control.
Moreover, when the object being photographed is located in a near distance (for example, 2-3 m), there often happens that a picture is over-exposed when the flashlight photography is taken at the full open diaphragm value. For example, the proper exposure is obtained at the guide number GN=2.8 under the condition that the full open F number is 1.4 and the object distance is 2 m. It is difficult to properly control such a small amount of the light emission from the electronic flash as a guide number GN=2.8.
The described technique which uniquely defines a diaphragm value in response to a charging complete signal fed from the electronic flash is applicable to the type of automatic electronic flash which controls the amount of emission in accordance with its controlling capability. By contrast, a so-called TTL automatic electronic flash was recently developed for use with a camera of the TTL direct photometry type and capable of photometry of light reflection from the film surface, which camera controls the emission of light from the electronic flash. In this instance, a camera may utilize any arbitrary value of diaphragm aperture rather than being fixed to any particular value. When so used, the electronic flash is activated to compensate for any deficiency of natural light at the time a shutter is fully open, and hence it is unnecessary to synchronize an exposure period with the operation of the electronic flash, representing a large departure from the emission control with a conventional electronic flash.
When TTL automatic electronic flash is used in combination with a camera of the TTL direct photometry type and capable of photometry of light reflection from the film surface, the electronic flash may emit light or may not emit light depending on the choice of a diaphragm aperture except when a very bright illumination is provided to an object being photographed. Specifically, when the diaphragm aperture is reduced, the exposure supplied by natural light decreases to result in an increased length of exposure period, so that synchro contacts are closed to cause the emission of light from the electronic flash when the shutter is fully open (synchronized with the operation of the electronic flash). Conversely, when the diaphragm aperture is increased, an increased amount of exposure supplied by natural light results in a reduced length of exposure period, failing to close the synchro contacts and thus permitting the photographing operation to be performed under natural light alone.
However, since the intended purpose of having the electronic flash to cooperate with the camera is to achieve an electronic flash effect upon the photographing operation, it is desired that a diaphragm control assures a reliable activation of the electronic flash while avoiding the loss of merits of a photographing operation effected with the aid of TTL automatic electronic flash, namely, while utilizing the entire range of diaphragm values which can be manually chosen on the part of the camera.
It is also necessary that such diaphragm control prevents a diaphragm value close to an open diaphragm from being established since then the depth of field is reduced to degrade the image quality and the emission control of a small amount of light results in an over-exposure of the object being photographed which is located in a near distance.